1. Field of the Invention
This invention relates to a method of manufacturing an aperture mask for a color cathode ray tube and more particularly to a method of producing an aperture mask comprised of an iron-nickel alloy and as incident to forming the blank with a desired final contour an annealing process is carried out to produce a tightly adhered black oxide coating on the surface of the metal while at the same time relieving forming stresses and restoring to a low value the coercive force properties of the alloy metal.
2. Description of the Prior Art
U.S. Pat. Nos. 4,210,843; 4,427,396; 4,609,412; and 4,536,226 disclose various processed for the manufacture of aperture masks for a color cathode ray picture tube. In the manufacture of an apertured mask for a color cathode ray tube, one material commonly selected for the aperture mask is a low carbon steel. Typically a 1008 grade of aluminum killed or rimmed carbon steel is used as the mask material. Recent requirement for higher image quality have brought about the desire to utilize the iron-nickel alloy, such as Invar, because of a low thermal expansion characteristic of this material which reduces the effect commonly known in the industry as doming. Doming is a deflection of the aperture mask in a direction toward the phosphor screen due to a thermal input produced by electrom beams impinging on the mask material. Thermal compensating clips are usually provided to adjust the spacing between the mask and the phosphor screen according to the thermal input, however, adequate to compensate for non-uniform heating of the mask material as, for example, when a limited area of the mask receive a greater thermal input than other areas of the mask. For example, such a limited area can be a white area on the screen displaying an image representing the playing surface of a hockey game. As is well known, a white area displayed by a color CRT tube is produced by an excitation of all three color phosphor deposits by impingement of all three electron beams through an aperture opening in the mask. When this occurs all three electron beams also impinge on the mask material immediately adjacent to the aperture opening.
U.S. Pat. Nos. 2,806,162 and 4,528,246 both disclose benefits gained through the use of an Invar type nickel-iron alloy material in the manufacture of an aperture mask for an color CRT. In U.S. Pat. No. 4,536,226 there is disclosed a method of manufacturing a shadow mask wherein a sheet of nickel-iron alloy such as Invar is treated by perforating a number of apertures after which the apertured sheet is annealed at a temperature of between 1652 degrees F. and 2192 degrees F. for a period of ten minutes in vacuum. The annealed sheet is pressed to form a shadow mask while the sheet is kept at a forming temperature of about 360 degrees F. The elevated temperature of the shadow mask during the forming operation is effective to reduce the yield strength of the alloy material.
It has been discovered that by using the high temperature anneal and the warm forming of the shadow mask, it is possible to at least theoretically reduce or eliminate spring back when the mask blank is formed into an aperture mask.